It is generally understood that ceramides present within the intercellular lipid lamellae of the stratum corneum have an important role in the production and maintenance of the water permeability barrier of the skin. Ceramides, or substances closely related to them, have been disclosed as components of skin care compositions. In particular, Kao Corporation in EP 0227994 and EP 0282816 disclose synthetic analogues of ceramides which, to a significant extent, have properties similar to natural ceramides, but are relatively cheaper to produce. However, the degree of skin benefit attributable to such synthetic ceramides or analogues thereof is limited to the extent that they do not fully mimic the natural ceramides of the skin, some of which contain N-acylated hydroxyfatty acids. Thus the general formula of molecules disclosed by Kao in EP 0227994 is structure (1): ##STR2## In JP-A-63-192703, Kao Corporation disclose a skin composition which contains extracted naturally occurring skin ceramides including either phytosphingosines or .alpha.-hydroxy fatty acid-containing ceramides. Synthetic hydroxylated ceramide structures are not disclosed. A further family of ceramides of the type found in skin, is disclosed in EP 097 059 (Unilever). This highlights the vital role played by .omega.-(O-linoleoyl) ceramides in the water barrier of the skin.
Fulmer & Kramer, in J. Invest. Derm. (1986) 86, 598-602, have observed that there is a relative deficiency of phytosphingosine-containing ceramide in detergent-induced dry skin conditions. Also, it is well documented that the stratum corneum water barrier function is impaired under such conditions (Tupker R.A. et al., Acta Derm. Venereol. Stockh [1990], 70, 1-5).
We have now discovered that the number of hydroxyl groups present within a ceramide structure is highly relevant to its influence on the water barrier properties. Furthermore, we have shown that synthetic hydroxylated ceramides, hereinafter referred to as "pseudoceramides" can be synthesised at lower cost than extracting the natural homologues from natural sources, and that these pseudoceramides posses properties necessary to improve water barrier function of the stratum corneum.